Modern commercial airplanes are equipped with a horizontal stabilizer tailplane whose inclination is adjustable. Such an adjustable horizontal tailplane is known in the art by one or the other of the abbreviations PHR (for Plan Horizontal Réglable [Adjustable Horizontal Trim] or THS (for “Trimmable Horizontal Stabilizer”). An actuator, controlled automatically during a normal flight in manual mode or under automatic pilot, or controlled manually by the pilot via a control provided in the cockpit in the case of operations on the ground or under fault conditions, permits adjustment of the inclination of the tailplane.
Several types of actuators were able to be provided in the art, such as the hydraulic actuator described in document WO 2007/028597.
In document EP 0983937, a traditional adjustable horizontal trim actuator is of the ballscrew/nut type (“ball screw/nut”), which offers good mechanical operating efficiency.
This screw/nut assembly achieves the mechanical linkage of variable length between the fixed structure of the airplane (the screw is connected to the fuselage by a ball-joint linkage) and the horizontal tailplane (integral with the nut). The rotation of the screw by a motor and associated gears causes translation of the nut and thus tilting of the horizontal tailplane around a horizontal axis of rotation, in one direction or the other depending on the inputs applied to the motor.
The objective of adjustment of the tailplane is to compensate for possible aerodynamic loads applied to this horizontal tailplane in order to guarantee the desired trajectory of the airplane while preventing the pilot from having to apply a compensating force continuously on the associated controls. In particular, this compensation is effected automatically by the flight control system.
The actuators are subjected to an aerodynamic load resulting from the distribution of loads in the airplane, from the trajectory of the latter and from the flight conditions. Thus, as illustrated by FIG. 1, this aerodynamic load F is transmitted, in our example, to the nut and then to screw 12 by way of balls. F is resolved into an axial component Fy and a radial component Fx.
The latter, Fx, causes progressive and undesired rotation of screw 12, which over time leads to a variation of the inclination of the horizontal tailplane and therefore of the trajectory of the airplane. The pilot must then readjust the displaced tailplane manually and frequently.
In order to avoid such an involuntary variation and to guarantee effective control of the adjustable horizontal trim, a no-back function of the actuator is provided.
In the case of an actuator of the screw/nut type, this function is assured by a no-back device, also known as “anti-return device”, “brake with anti-displacement system” or “no-back brake”. This no-back device is integrated in the actuator and may be redundantly designed, as shown in document EP 0983937.
More generally, an electromechanical actuator (EMA) may be equipped with a no-back device, such as with the monitoring device of document EP 1245867.
In connection with an analysis of airplane safety, the integrity of the no-back function of the THS actuators (or “THSA”) is verified.
At present, this verification of the integrity of the “no-back brake” device is achieved by periodic operational tests on the airplane, particularly on the THS actuator.
In fact, because this no-back device is integrated in the actuator, a functional test aimed at directly verifying the integrity of the no-back function of the actuator is not conceivable. It is therefore necessary to resort to a substantially subjective operational test.
The test is based on characteristics specific to the no-back device, and consists, for an operator, in listening, during startup of the system by means of an input and of force applied to the horizontal tailplane, for a specific and repetitive “tick-tick” noise, which is normally emitted by the device during operation.
In addition to the subjective and therefore poorly robust nature of this test, other disadvantages of this test exist:
it is laborious, because it requires several operators simultaneously and a long downtime for the airplane;
consequently, it is performed infrequently (after several hundred or thousands of flying hours): a failure may therefore remain concealed during the interval between two successive tests. In the safety analysis of the airplane, this low frequency is compensated for by weighting of the hypothetical failure rate of the brake device. On this basis a statistical breakdown of the probability of failure of the device is obtained;
it covers a limited range of possible failures, while other failure modes are not detectable by the “tick-tick” test. The failures of these other modes then tend to remain concealed during the lifetime of the airplane.